For adapting to an optical communication system operating at high speed, high sensitivity and high reliability semiconductor light receiving devices are required to be able to operate at a wavelength of 1.3 to 1.6 .mu.m at high speed, high gain-bandwidth (GB) product, low dark current and high reliability. On pages 38 to 40, Volume 40(1), Appl. Phys. Lett. 1982, by F. Capasso et al., an invention by which ionization rate ratio .alpha./.beta. is increased by using conduction band-edge discontinuity .DELTA.Ec of superlattice for impact ionization of electrons which has been proposed. 0n pages 1419 to 1423, J. Quantum Electronics, Volume 28(6), 1992, by Kagawa et al., an invention by which ionization rate ratio .alpha./.beta. is increased by using an InGaAsP/InAlAs system of superlattice with light sensitivity in a wavelength of 1.3 to 1.6 .mu.m for long distance optical communication has been proposed. The ionization rate ratio .alpha./.beta. is increased up to 5 in contrast to that up to 2 by Bulk InGaAs.
According to the superlattice structure, conduction band-edge discontinuity .DELTA.Ec is 0.39 eV which is larger than 0.03 eV of valence band-edge discontinuity .DELTA.Ev, so that electrons have more energy than holes by band-edge discontinuity when the electrons and holes get in a well layer. That means electrons reach the threshold energy of ionization, whereby the ionization rate ratio is increased and low noise operation can be realized.
With the above mentioned avalanche photodiode, however, leakage current is generated due to the interface level between semiconductor layers (multiplication layer, field buffer layer and light absorption layer) on the side surface of a mesa structure and an SiN passivation layer, and a residual oxide layer and defects on the surface. The leakage current causes dark current to increase from 0.8 .mu.A up to several .mu.A in a practical multiplication factor of 10 to 20, whereby more noise is generated. The passivation boundary surface is unstable at an atmospheric temperature of 200.degree. C. and reverse current of 100 .mu.A, which are the general conditions reliability tests.
Nakamura et al. fabricated a superlattice APD (Avalanche Photo Diode) using a polyimide layer as a mesa passivation layer, on pages 261 to 264, TuC5-4, ECOC, 1991. In this structure, there are many interface level (more than 2.times.10.sup.12 cm.sup.-2 eV), on the boundary surface between the polyimide layer and semiconductor layers (multiplication layer, field buffer layer and light absorption layer) on the side surface of the mesa structure. This kind of interface level generally is caused by dangling bond on the boundary surface between semiconductor and polyimide layer, between the semiconductor native oxide layer and semiconductor, and on surface defects, etc. Generally, in the semiconductor layers depleted in reverse bias, the first dangling bond tends to be generated in a p InGaAs light absorption layer having a relatively smaller forbidden bandgap, and the last dangling bond tends to be generated in a superlattice multiplication layer including aluminum atomics which can be natural-oxidized easily. In a high electric field (500 to 600 kV/cm), .mu.A order of surface-leakage dark current is generated based on those interface levels. As time goes on, the interface levels and surface defects are increased due to the effect of hot-carrier injection into the passivation layer, and therefore, dark current is increased. In mesa type of photodiodes in which the semiconductor depletion layer is exposed on the side surface, surface leakage current is large and becomes unstable as time goes on.
With a planar type of device, described in a Japanese Patent Publication Kokai H4-10478, p.sup.- type material having a carrier concentration of less than 10.sup.16 cm.sup.-3 is doped into a superlattice multiplication layer, however, it is difficult to realize this kind of doping into mixed crystal by general crystal growth method precisely.